Electrochemical capacitors (ECs), also known as ultracapacitors or supercapacitors, are being considered for uses in hybrid electric vehicles (EVs) where they can supplement a battery used in an electric car to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but capacitors (with their ability to release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. The EC must also store sufficient energy to provide an acceptable driving range. To be cost- and weight-effective compared to additional battery capacity they must combine adequate specific energy and specific power with long cycle life, and meet cost targets as well. Specifically, they must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, and provide high cycle-life (>100,000 cycles).
ECs are also gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. For a given applied voltage, the stored energy in an EC associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1 nm, thus forming an extremely small effective “plate separation.” In some ECs, stored energy is further augmented by pseudo-capacitance effects, occurring again at the solid-electrolyte interface due to electrochemical phenomena, such as the redox charge transfer. The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at electrode-electrolyte interfaces providing high capacitance.
Experience with ECs based on activated carbon electrodes shows that the experimentally measured capacitance is always much lower than the geometrical capacitance calculated from the measured surface area and the width of the dipole layer. For very high surface area carbons, typically only about ten percent of the “theoretical” capacitance was observed. This disappointing performance is related to the presence of micro-pores and ascribed to inaccessibility of some pores by the electrolyte, wetting deficiencies, and/or the inability of a double layer to form successfully in pores in which the oppositely charged surfaces are less than about 2 nm apart. In activated carbons, depending on the source of the carbon and the heat treatment temperature, a surprising amount of surface can be in the form of such micro-pores.
It would be desirable to produce an EC that exhibits greater geometrical capacitance using a carbon based electrode having a high accessible surface area, high porosity, and reduced or no micro-pores. It would be further advantageous to develop carbon-based nano-structures that are conducive to the occurrence of pseudo-capacitance effects such as the redox charge transfer.
Carbon nanotubes (CNT) are nanometer-scale sized tube-shaped molecules having the structure of a graphite molecule rolled into a rube. A nanotube can be single-walled or multi-walled, dependent upon conditions of preparation. Carbon nanotubes typically are electrically conductive and mechanically strong and stiff along their length. Nanotubes typically also have a relatively high aspect ratio (length/diameter ratio). Due to these properties, the use of CNTs as reinforcements in composite materials for both structural and functional applications would be advantageous. In particular, CNTs are being studied for electrochemical supercapacitor electrodes due to their unique properties and structure, which include high surface area, high conductivity, and chemical stability. Capacitance values from 20 to 180 F/g have been reported, depending on CNT purity and electrolyte, as well as on specimen treatment such as CO2 physical activation, KOH chemical activation, or exposure to nitric acid, fluorine, or ammonia plasma. Conducting polymers, such as polyacetylene, polypyrrole, polyaniline, polythiophene, and their derivatives, are also common electrode materials for supercapacitors. The modification of CNTs with conducting polymers is one way to increase the capacitance of the composite resulting from redox contribution of the conducting polymers. In the CNT/conducting polymer composite, CNTs are electron acceptors while the conducting polymer serves as an electron donor. A charge transfer complex is formed between CNTs in their ground state and aniline monomer. A number of studies on CNT/conducting polymer composites for electrochemical capacitor applications have been reported. The following references are related to this subject:    1. K. H. An, et al., “Electrochemical Properties of High-Power Supercapacitors Using Single-Walled CNT Electrodes,” Advanced Functional Materials, 11 (No. 5) (October 2001) 387-392.    2. G. Z. Chen, “Carbon Nanotube and Polypyrrole Composites: Coating and Doping,” Advanced Materials, 12 (No. 7) (2000) 522-526.    3. C. Zhou, et al., “Functionalized Single Wall CNTs Treated with Pyrrole for Electrochemical Supercapacitor Membranes,” Chemistry of Materials, 17 (2005) 1997-2002.    4. K. Jurewicz, et al., “Supercapacitors from Nanotubes/Polypyrrole Composites,” Chemical Physics Letters, 347 (October 2001) 36-40.    5. J. E. Huang, et al., “Well-dispersed Single-walled CNT/Polyaniline Composite Films,” Carbon, 41 (2003) 2731-2736.    6. H. Tennent, et al., “Graphitic Nano-fibers in Electrochemical Capacitors,” U.S. Pat. No. 6,031,711 (Feb. 29, 2000).
However, there are several drawbacks associated with carbon nanotube-filled composites. First, CNTs are known to be extremely expensive due to the low yield and low production and purification rates commonly associated with the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Second, it is well-known in the field of composites that the reinforcement fiber orientation plays an important role in governing the mechanical and other physical properties of a composite material. Unfortunately, CNTs tend to form a tangled mess resembling a hairball, which is difficult to work with. This tendency and other difficulties have limited efforts toward realizing a composite material containing well-dispersed CNTs with desired orientations.
Instead of trying to develop much lower-cost processes for making CNTs, researchers (Jang, et al.) at Nanotek Instruments, Inc., have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but are more readily available and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-sized graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. FIG. 1 shows an atomic force microscopic picture of a sample of NGPs. In practice, NGPs are obtained from a precursor material, such as minute graphite particles, using a low-cost process, but not via flattening of CNTs. These nano materials could potentially become cost-effective substitutes for CNTs or other types of nano-rods for various scientific and engineering applications. These diligent efforts have led to the following patent or patent applications:    7. B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006).    8. B. Z. Jang, L. X. Yang, S. C. Wong, and Y. J. Bai, “Process for Producing Nano-scaled Graphene Plates,” U.S. patent Ser. No. 10/858,814 (Jun. 3, 2004).    9. Jiusheng Guo, A. Zhamu, and B. Z. Jang, “Nano-scaled Graphene Plate-Reinforced Composite Materials and Method of Producing Same,” U.S. patent Ser. No. 11/257,508 (Oct. 26, 2005).    10. Lulu Song, Jiusheng Guo, A. Zhamu, and Bor Z. Jang, “Highly Conductive Nano-scaled Graphene Plate Nanocomposites and Products” U.S. patent Ser. No. 11/328,880 (Jan. 11, 2006).
Specifically, Jang, et al. [Ref.8] disclosed a process to readily produce NGPs in large quantities. The process includes the following procedures: (1) providing a graphite powder containing fine graphite particles preferably with at least one dimension smaller than 200 μm (most preferably smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled, resulting in the formation of NGPs with a platelet thickness smaller than 100 nm. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity)) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width, and length values. We have successfully prepared NGPs with an average length smaller than 500 nm and, in several cases, smaller than 100 nm. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important engineering applications. The electronic, thermal, and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes, but NGPs will be available at much lower costs and in larger quantities.
The NGP material can be used as a nano-scaled reinforcement for a matrix material to obtain a nanocomposite. Expected advantages of nano-scaled reinforcements in a matrix material include: (1) when nano-scaled fillers are finely dispersed in a polymer matrix, the tremendously high surface area could contribute to polymer chain confinement effects, possibly leading to a higher glass transition temperature, stiffness and strength; (2) nano-scaled fillers provide an extraordinarily zigzagging, tortuous diffusion path that results in enhanced barrier or resistance against permeation of moisture, oxygen, other gases, and liquid chemical agents. Such a tortuous structure also serves as an effective strain energy dissipation mechanism associated with micro-crack propagation in a brittle matrix such as ceramic, glass, or carbon; (3) nano-scaled fillers can also enhance the electrical and thermal conductivities in a polymer, ceramic or glass matrix; and (4) carbon-based nano-scaled fillers have excellent thermal protection properties, and, if incorporated in a matrix material, could potentially eliminate the need for a thermal protective layer, for instance, in rocket motor applications.
In a related subject, exfoliated graphite may be impregnated with a resin to obtain an expanded graphite flake (EGF)-resin composite. Alternatively, expandable graphite particles may be dispersed in a monomer or oligomer and then exfoliated before the monomer/oligomer is polymerized or cured, also resulting in the formation of an expanded graphite flake-resin composite. These conventional exfoliated graphite flake composites are discussed in the following references:    11. M. Xiao, L. Y. Sun, J. J. Liu, Y. Li, and K. C. Gong, “Synthesis and Properties of Polystyrene/Graphite Nanocomposite,” Polymer, 43-8 (2002) 2245.    12. G. H. Chen, C. Wu, W. Weng, D. Wu, and W. Yan, “Preparation of Polystyrene/Graphite Nano-sheet Composite,” Polymer, 44 (2003) 1781-1784.    13. W. Zheng, S. C. Wong, and H. J. Sue, “Transport behavior of PMMA/expanded graphite nanocomposites,” Polymer, 73 (2002) 6767.    14. W. Zheng and S. C. Wong, “Electrical conductivity and dielectric properties of PMMA/expanded graphite composites,” Composite Sci., and Tech., 63 (2003) 225.    15. L. T. Drzal and H. Fukushima, “Expanded Graphite and Products Produced Therefrom,” U.S. patent application Ser. No. 10/659,577 (Sep. 10, 2003).    16. L. R. Bunnell, Sr., “Enhancement of the Mechanical Properties by Graphite Flake Addition,” U.S. Pat. No. 4,987,175 (Jan. 22, 1991).    17. L. R. Bunnell, Sr., “Method for Producing Thin Graphite Flakes with Large Aspect Ratios,” U.S. Pat. No. 5,186,919 (Feb. 16, 1993).
The application of these prior-art composites have been limited to electrical and mechanical applications, but not for supercapacitor electrode applications. For instance, thin exfoliated graphite flakes, with extremely high aspect ratio (length/thickness ratio>100˜1000), lead to a lower percolation threshold (typically 1-4% by weight EGF) for forming an electron-conducting path, as compared to a threshold of typically 5-20% for other types of graphite particles. However, conventional EGF composites with a high EGF loading either cannot be formed into thin composite plate, cannot be molded with mass production techniques, or are simply not processable into useful products. Although one would expect the electrical conductivity of an EGF composite to become higher if the EGF loading is greater (e.g., >20% by weight), no melt-blended composite containing more than 20% by weight of well-dispersed, fully separated EG flakes has hitherto been reported. The approach of “intercalation and in situ polymerization” is applicable to only a limited number of polymers that have a wide window of synthesis conditions such as polystyrene and nylon-6.
After an extensive and in-depth study of the electrochemical response of both isolated and fully separated EGFs and NGPs and their composites, we have found that a certain class of meso-porous composites containing EGFs and NGPs as electrode ingredients exhibit superior charge double layer-type supercapacitance and redox charge transfer-type pseudo-capacitance. These electrode materials can be mass-produced cost-effectively and, hence, have much greater utility value compared to carbon nanotube-based materials.
Thus, it is an object of the present invention to provide a porous nanocomposite that contains fully separated graphite platelets with a sufficient amount and packing arrangement effective for achieving a high surface area greater than 100 m2/gm (typically greater than 200 m2/gm, some greater than 500 m2/gm, and even greater than 1000 m2/gm when the nanocomposite matrix is made through pyrolization of a polymer).
It is another object of the present invention to provide a porous nanocomposite that contains fully separated graphite platelets with a sufficient level of porosity effective for achieving a high capacitance value when used as a supercapacitor electrode.
It is yet another object of the present invention to provide a porous nanocomposite electrode comprising fully separated graphite platelets that are smaller than 10 μm in length, width or diameter (typically and preferably smaller than 0.5 μm or 500 nm) and smaller than 100 nm in thickness (typically and preferably smaller than 10 nm).
It is still another object of the present invention to provide a porous nanocomposite comprising fully separated graphite platelets that are surface-functionalized or activated.
It is still another object of the present invention to provide a porous nanocomposite electrode comprising fully separated graphite platelets that are smaller than 10 μm in length, width or diameter (preferably smaller than 0.5 μm or 500 nm) and smaller than 100 nm (preferably smaller than 10 nm) in thickness. These nano-scaled graphene plates are attached to or bonded by a conductive material such as a conjugate chain polymer for a significantly improved charge storage capacity. The matrix material may comprise a conducting polymer, polymeric carbon, coal tar pitch, petroleum pitch, glassy or amorphous carbon, or a combination thereof